Collect. Czech. Chem. Commun. 1994, 59, 381-390

Chemometric Analysis of Substituent Effects. II. Relation Between Hammett Substituent Constants σm and σp and a New Model for Quantitative Description of Substituent Effects

Oldřich Pytela

Department of Organic Chemistry, University of Chemical Technology, 532 10 Pardubice, Czech Republic


The paper presents values of 25 substituent constants σi obtained by optimizing 46 data series of dissociation constants of substituted benzoic acids in various media. The constants σi fulfil the general relation between the substituent constants of the Hammett type in meta and para positions enabling the description of substituent effects from both positions at the same time by a single constant. The Hammett substituent constants are interpreted by means of the σi constants with an accuracy better than 0.03 units. In addition to it, the validity of general relationship between σp and σm was verified on a set of 56 substituents with the prediction accuracy of 0.06 units for σp, and after excluding the probably incorrectly parametrized substituents NHCOC6H5, CH3S, and F the accuracy has improved to 0.05 units (98% of interpreted variability). The given relationship has served as a basis for suggesting a new model of transfer of substituent effects to a reaction centre: the model involves both the Hammett equation and the Yukawa-Tsuno equation and explains their background. The suggested model uses generalized transmission coefficients to separately describe the transformation of a single primary substituent effect - depending on its structure - into one inductive and two resonance effects which are transmitted through two independent channels to the reaction centre and here transformed into the resulting observable effect. From the model it follows that the substituent constant σp is not a substituent constant in the true sense of the word since it involves the characteristics of skeleton and of reaction centre.